Support for tax_exempt in draft orders

[akipnis-pvolve]

New field to allow setting tax_exempt on draft orders (see DraftOrder properties under https://shopify.dev/docs/admin-api/rest/reference/orders/draftorder).

From documentation:

Whether taxes are exempt for the draft order. If set to false, then Shopify refers to the taxable field for each line_item. If a customer is applied to the draft order, then Shopify uses the customer's tax_exempt field instead.

Field is *bool to distinguish between setting value to false or leaving value unchanged for updates.

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (14be5c1) compared to base (49ba021). Patch has no changes to coverable lines.

:exclamation: Current head 14be5c1 differs from pull request most recent head c0999ea. Consider uploading reports for the commit c0999ea to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #163 +/- ## ========================================== Coverage 100.00% 100.00% ========================================== Files 38 37 -1 Lines 1537 1255 -282 ========================================== - Hits 1537 1255 -282 ``` | [Impacted Files](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce) | Coverage Δ | | |---|---|---| | [draft\_order.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-ZHJhZnRfb3JkZXIuZ28=) | `100.00% <ø> (ø)` | | | [blog.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-YmxvZy5nbw==) | `100.00% <0.00%> (ø)` | | | [page.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-cGFnZS5nbw==) | `100.00% <0.00%> (ø)` | | | [shop.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-c2hvcC5nbw==) | `100.00% <0.00%> (ø)` | | | [util.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-dXRpbC5nbw==) | `100.00% <0.00%> (ø)` | | | [asset.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-YXNzZXQuZ28=) | `100.00% <0.00%> (ø)` | | | [image.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-aW1hZ2UuZ28=) | `100.00% <0.00%> (ø)` | | | [oauth.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-b2F1dGguZ28=) | `100.00% <0.00%> (ø)` | | | [order.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-b3JkZXIuZ28=) | `100.00% <0.00%> (ø)` | | | [theme.go](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce#diff-dGhlbWUuZ28=) | `100.00% <0.00%> (ø)` | | | ... and [29 more](https://codecov.io/gh/bold-commerce/go-shopify/pull/163?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bold+Commerce)

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[oliver006]

Thanks for your PR! I recently joined as a maintainer and so PRs should move a bit quicker again.